For toilet-bowl cleansing, aesthetic and water conservation (masking urine color) reasons, there is widespread use of a colored (usually blue) slow dissolving water coloring toilet bowl cleanser in a glass jar that is placed on the bottom of a toilet flush tank. There are many long periods of non-flushing as in week-end and summer homes; homes vacant during week-end and summer. Chemical placed in flush tank is messy once dissolution takes place; therefore, it is left to waste in the tank during these long periods of non-flushing. When its action is not needed or desired there is no convenient way to stop the dissolving action. This invention provides a simple means of optionally stopping and starting this water treatment process without lifting of the tank lid which is commonly a fully loaded platform for bathroom supplies and accessories. Because of the off and on control of this invention it is an excellent colorimetric detector of leaks past the flush valve and loss of water past the overflow pipe of a toilet flush tank. This device utilizes the fact that a fishing line is strong and of small diameter so that, when a toilet-tank lid is supported just enough to clear movement of the line across rim of a flush-tank, position of a loose body of chemical connected to the line within tank can be moved in and out of tank water by manipulation of the end of the line which is inside of the tank.